SC4530 datasheet(11/23 Pages) SEMTECH

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SC4530

Applications Information (Continued)

Output Capacitor Selection

The output ripple voltage ∆V

OUT of a step-down regulator

in continuous conduction can be expressed as:

OUT

OFF

OUT

ESR

(8)

where C

OUT is the output capacitance.

The first term in Equation (8) results from the equivalent

series resistance (ESR) of the output capacitor while the

rest is due to the charging and discharging of C

OUT by the

inductor ripple current.

Substituting ∆I

L = 50mA, tON + tOFF = 2µs, COUT = 22µF and

ESR = 3mΩ in Equation (8), we get:

)

(

OUT

Depending on the switching period and the type of the

capacitor used, the output voltage ripple resulting from

charging/discharging of C

OUT may be higher than the ripple

due to the ESR. The example above also shows that the

output voltage ripple in continuous mode is very low.

The SC4530 relies on fast amplifier response to reduce the

output voltage overshoot during power-up. Neither the

error amplifier output nor the reference is ramped during

start-up. The Zener diode D

Z (refer to page 5) clamps

the amplifier output, while the regulator output voltage

ramps up. As a result, the switch Q

is turned off every

cycle at the switch current limit, I

LIM (typically 0.5A). The

regulator thus delivers about 0.5A to its output until V

OUT

rises to its set value. If the load is light, then the amplifier

output voltage will fall below the RUN/IDLE threshold

following regulation. This causes the regulator to idle.

However the energy previously stored in the inductor

still flows to the output, causing the output voltage to

rise above its regulation level. The minimum output

capacitance required to keep the overshoot to less than

% of the nominal output voltage is:

OUT

LIM

OUT

(9)

The minimum output capacitance for various output

voltages can be estimated from Equation (9) using the

inductances given in Table 2. The results are shown in

Table 3. Smaller output capacitors may also be used if

higher output voltage overshoot is acceptable.

Table 3. Calculated Minimum Output Capacitance for 1%

OUT Overshoot during Start-up

OUT (V)

Minimum C

OUT (µF)

IN = 6V

IN = 30V

2.5

3.3

5.0

5.6

3.7

Ceramic capacitors are the best choice for most

applications. Sanyo TPE series polymer capacitors in B-

case, which offer large capacitors (>00µF) with slightly

higher ESR, are also good alternatives. Ripple current in

the output capacitor is not a concern because the inductor

current of a step-down converter directly feeds C

OUT,

resulting in very low ripple current. Avoid using Z5U or

Y5V ceramic capacitors because these types of capacitors

have high temperature and high voltage coefficients.

Bootstrapping the Power Transistor

To reduce the switch on-state voltage and maximize

efficiency, the base of the power transistor should be

driven from a power supply higher in voltage than V

IN. The

required driver supply voltage (at least 2V higher than the

SW) is generated with a bootstrap capacitor C

3 connected

between the BST and the SW nodes (Figure ) and the

bootstrap diode D

2 (Figure 2). The D2 anode is connected

to the BIAS pin.

During startup, the power transistor in the SC4530 is first

switched on so the current flows through to the inductor.

When the transistor is switched off, the inductor current

pulls the SW voltage low, allowing C

3 to be charged

through the internal bootstrap diode D

2. When the power

switch is turned on again, the SW voltage goes high. This

brings the BST voltage to V

SW + VC3, thus back-biasing D2.

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